Engine-driven machines such as, for example, dozers, loaders, excavators, motor graders, and other types of heavy equipment typically include a cooling system that cools the associated engine and other machine components below a threshold that provides for longevity of the machines. The cooling system consists of one or more air-to-air and/or liquid-to-air heat exchangers that chill coolant circulated throughout the engine and combustion air directed into the engine. Heat from the coolant or combustion air is passed to air from a fan that is speed controlled based on a temperature of the engine.
The cooling system fan is generally hydraulically powered. That is, a pump driven by the engine draws in low-pressure fluid and discharges the fluid at elevated pressures to drive a motor that is mechanically connected to the fan. When a temperature of the engine is higher than desired, the pump and motor work together to increase the speed of the fan. When the temperature of the engine is low, the pump and motor work together to decrease the speed of the fan and, in some situations, even stop the fan altogether.
Although effective at cooling the engine, it has been found that the hydraulic circuit driving the cooling fan described above and/or other hydraulic circuits of the same machine may have excess capacity at times that is not utilized or even wasted. With increasing focus on the environment, particularly on machine fuel consumption, it has become increasingly important to fully utilize all resources.
One attempt to improve hydraulic circuit efficiency is described in U.S. Pat. No. 6,460,332 that issued to Maruta et al. on Oct. 8, 2002 (“the '332 patent”). Specifically, the '332 patent discloses a hydraulic circuit that includes a pump connected to a motor in an open-loop circuit. An accumulator is disposed between the pump and motor and configured to accumulate fluid pressurized by the pump and discharge accumulated fluid to the motor.
Although the accumulator of the '992 patent may help to more fully utilize available resources, it may also be limited. That is, the system of the '992 patent does not provide a way to unload the pump during discharge of the accumulator. Without this ability, any benefit provided by the accumulator may not be fully realized. In addition, the configuration of the '992 patent may be limited from different types of circuits, for example from a cooling fan circuit.
The disclosed hydraulic circuit is directed to overcoming one or more of the problems set forth above and/or other problems of the prior art.